The art of inkjet printing technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ inkjet technology for producing hard copy printed output. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985) Vol. 39, No. 4 (Aug. 1988) Vol. 39, No. 5 (Oct. 1988), Vol. 43, No. 4 (Aug. 1992) Vol. 43, No. 6 (Dec. 1992) and Vol. 45 No. 1 (Feb. 1994) editions. Inkjet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
With the increased popularity of inkjet printing comes the increased competition between manufacturers of inkjet printers. Currently, the most common distinguishing features between competitive inkjet printers are price, speed, and print quality. Today, most inkjet printer manufacturers sell a model of printer in each price range of inkjet printers ranging from low cost home and office printers to high-speed commercial printers. To be competitive within each price range, the printer manufacturer must supply a printer with a faster print speed and a better resultant print quality than his competitors'. With price at a consistent low among the competitive inkjet printer manufacturers, a fast print speed directly coupled to a superior resultant print quality is key to the consumer's selection.
In many systems, color is mapped between that viewed on the monitor or display in a RGB (Red, Green, Blue) format to that outputted by an inkjet printer in a CMY (Cyan, Magenta, Yellow) format. This color mapping is necessary to obtain the “true” colors the user expects based on the display of color on the monitor as the mixing of the three colors are different between RGB and CMY schemes. Other color mapping processes are required to map colors between other color schemes; for example, between an image source file containing CMYK data and the CMYK color space of the chosen printer. U.S. Pat. No. 5,704,021 to Smith et al., assigned to the same Assignee as the present invention, discloses the process of “color mapping” in color inkjet printers, and is incorporated by reference in its entirety. Color mapping provides the printer driver with the information defining the combination and amount of color ink droplets to be applied to get the requested color.
However, due to the fixed ordering of the color print cartridges in the printer carriage, the order in which the ink droplets are to be applied to the print medium is fixed, for a given direction of travel of the print carriage. For example, in an inkjet printer where the print cartridges are ordered CMY, when traversing from left to right, a combination of Cyan and Yellow will have to be ordered Yellow then Cyan. In the event the inkjet printer works in a bidirectional mode, when returning in the right to left direction, a combination of Cyan and Yellow will have to be ordered Cyan then Yellow. This typically creates a varying color between left to right passes and right to left passes of the print cartridges. This variation may produce an undesirable banding, or hue shift effect in the output, as is well understood in the art.
The fastest way to print a contiguous area of color with a scanning inkjet printhead is to sweep the printhead across the media in a first direction while firing ink droplets as prescribed by the color map from an array of nozzles, advance the media the height of the array of nozzles then sweep the printhead in a second, opposite direction firing ink droplets as before. This is known as single-pass, bidirectional printing. Single-pass because the printhead passes over each area of the page only one time. There is minimal or no overlap between adjacent printed rows. Bi-directional because drops are fired while the printhead is travelling in both the left to right direction and the returning right to left direction.
This technique is well known and successful for printing in monochrome. Workers skilled in this field have recognized, however, that for printing in color a hue shift, or more precisely a color hue shift, arises between printing left to right over right to left.
One solution to this problem has been to digress to single-pass unidirectional printing. In this case, everything is printed in a left to right order, thereby eliminating the directional related color hue shifts. A second solution to the problem is to resort to multi-pass bi-directional printing where the colors are overlayed in an averaging or blending scheme whereby the banding becomes less noticeable. Either of their two solutions compromise print speed to achieve highest print quality.
A third solution is disclosed in U.S. Pat. application Ser. No. 09/303249 to Ross, filed on Apr. 13, 1999 and assigned to the same Assignee as the present invention, which is entitled “Method of minimizing hue shifts in a single-pass, bi-directional inkjet printer using direction dependent color maps”. This patent discloses a method of employing two separate color maps for use in bi-directional printing and is incorporated by reference in its entirety. One color map is used for printing when the printer carriage is moving in a left to right print direction and the second color map is used when the printer carriage is moving in a right to left print direction. The use of two color maps allows the quantity and placement of individual ink droplets to be adjusted, according to their order of application, to minimize the directional related color hue shift.
Although this solution gives rise to significant improvements over the prior art, there remains room for further refinement. One reason for this is that the suitability of the two color maps depends upon various criteria that are subject to change. For example, the ambient environmental conditions and characteristics of the printer and the media being used may each affect a viewer's perception of a color printed and thus the effectiveness of a given pair of color maps.
With the increasing use of inkjet printers for high quality color printing in the home and in the office, there is a need for a high speed, low cost, compact inkjet printer that produces a uniform high quality output. It would therefore be desirable to provide a method and apparatus which addresses the problems of the prior art.